Electrically gated three-terminal circuits and devices based on spin hall torque effects in magnetic nanostructures apparatus, methods and applications

ABSTRACT

3-terminal magnetic circuits and devices based on the spin-transfer torque (STT) effect via a combination of injection of spin-polarized electrons or charged particles by using a charge current in a spin Hall effect metal layer coupled to a free magnetic layer and application of a gate voltage to the free magnetic layer to manipulate the magnetization of the free magnetic layer for various applications, including non-volatile memory functions, logic functions and others. The charge current is applied to the spin Hall effect metal layer via first and second electrical terminals and the gate voltage is applied between a third electrical terminal and either of the first and second electrical terminals. The spin Hall effect metal layer can be adjacent to the free magnetic layer or in direct contact with the free magnetic layer to allow a spin-polarized current generated via a spin Hall effect under the charge current to enter the free magnetic layer. The disclosed 3-terminal magnetic circuits can also be applied to signal oscillator circuits and other applications.

PRIORITY CLAIM AND RELATED PATENT APPLICATION

This patent document is a 35 USC 371 National Stage application ofInternational Application No. PCT/US2013/053874 filed Aug. 6, 2013,which claims the priority of U.S. Provisional Application No. 61/679,890entitled “ELECTRICALLY GATED MODULATION OF SPIN HALL TORQUE EFFECTS INMAGNETIC NANOSTRUCTURES” and filed on Aug. 6, 2012, the entirety ofwhich is incorporated by reference as part of the disclosure of thispatent document.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with support from the United States governmentunder DOD/DARPA grant HR0011-11-C-0074, Navy/ONR grant N00014-10-1-0024and Army/ARO grant W911NF-08-2-0032. The government has certain rightsin the invention.

TECHNICAL FIELD

This patent document relates to circuits and devices having magneticmaterials or structures based on electron spin torque effects and theirapplications, including non-volatile magnetic memory circuits,non-volatile magnetic logic devices, and spin-torque excited nanomagnetoscillators.

BACKGROUND

Electrons and other charged particles process spins as one of theirintrinsic particle properties and such a spin is associated with a spinangular momentum. A spin of an electron has two distinctive spin states.Electrons in an electrical current may be unpolarized by having theequal probabilities in the two spin states. The electrons in anelectrical current are spin polarized by having more electrons in onespin state than electrons in the other spin state. A spin-polarizedcurrent can be achieved by manipulating the spin population via variousmethods, e.g., by passing the current through a magnetic layer having aparticular magnetization. In various magnetic microstructures, aspin-polarized current can be directed into a magnetic layer to causetransfer of the angular momenta of the spin-polarized electrons to themagnetic layer and this transfer can lead to exertion of a spin-transfertorque on the local magnetic moments in the magnetic layer andprecession of the magnetic moments in the magnetic layer. Under a propercondition, this spin-transfer torque can cause a flip or switch of thedirection of the magnetization of the magnetic layer.

The above spin-transfer torque (STT) effect can be used for variousapplications including STT magnetic random access memory (MRAM) circuitsand devices. For example, as illustrated in FIG. 1, a STT-MRAM circuitcan include a magnetic tunnel junction (MTJ) as a magnetoresistiveelement formed of two or more thin film ferromagnetic layers orelectrodes, which are usually referred to as the free magnetic layer(FL) having a magnetic moment that can be switched or changed and thepinned magnetic layer (PL) whose magnetic moment is fixed in direction.The free magnetic layer (FL) and the pinned magnetic layer (PL) areseparated by an insulating barrier layer (e.g., a MgO layer) that issufficiently thin to allow electrons to transit through the barrierlayer via quantum mechanical tunneling when an electrical bias voltageis applied between the electrodes. The electrical resistance across theMTJ depends upon the relative magnetic orientations of the PL and FLlayers. The magnetic moment of the FL can be switched between two stableorientations in the FL. The resistance across the MTJ exhibits twodifferent values under the two relative magnetic orientations of the PLand FL layers, which can be used to represent two binary states “1” and“0” for binary data storage, or, alternatively, for binary logicapplications. The magnetoresistance of this element is used to read outthis binary information from the memory or logic cell.

In various STT-MRAM and other circuits, the MTJ is a 2-terminal MTJcircuit that directs a current from one terminal through the tunnelbarrier to the other terminal FIG. 1 further illustrates this 2-terminalcircuit configuration where a 2-terminal control circuit is coupled tothe terminals on two sides of the MTJ. In a write operation, the2-terminal control circuit sends a sufficiently large write current in aselected current flow direction from one terminal through the barrierlayer to the other terminal through the barrier layer to set themagnetic orientation of the free layer relative to the reference layerrepresenting a desired binary state. In a read operation, the 2-terminalcontrol circuit uses the same two terminals to send a read current whichis less than the large write current through the barrier layer tomeasure the resistance across the MTJ corresponding to a stored bitunder a particular relative magnetic orientation of the PL and FLlayers.

SUMMARY

The techniques and devices disclosed in this document provide 3-terminalmagnetic circuits and devices based on the spin-transfer torque (STT)effect via a combination of injection of spin-polarized electrons orcharged particles by using a charge current in a spin Hall effect metallayer coupled to a free magnetic layer and application of a gate voltageto the free magnetic layer to manipulate the magnetization of the freemagnetic layer for various applications, including non-volatile memoryfunctions, logic functions and others. The charge current is applied tothe spin Hall effect metal layer via first and second electricalterminals and the gate voltage is applied between a third electricalterminal and either of the first and second electrical terminals. Thespin Hall effect metal layer can be adjacent to the free magnetic layeror in direct contact with the free magnetic layer to allow aspin-polarized current generated via a spin Hall effect under the chargecurrent to enter the free magnetic layer. The disclosed 3-terminalmagnetic circuits can also be applied to signal oscillator circuits andother applications.

A magnetic tunnel junction (MTJ) memory cell can be constructed in a3-terminal circuit configuration for non-volatile magnetic memoryapplication and can be operated to use the combined operation of thecharge current in the spin Hall effect metal layer and the gate voltageto the free magnetic layer to effectuate the magnetization switching ofthe free magnetic layer in a write operation. The reading of the MTJmemory cell can be done by applying a read voltage across the MTJ.

A magnetic tunnel junction (MTJ) in this 3-terminal circuitconfiguration can also be used to form a signal oscillator based on themagnetic precession in the free magnetic layer caused by the spin torquecaused by the spin-polarized current induced by the charge current inthe spin Hall effect metal layer and a sensing current can be appliedacross the MTJ to be modulated by the oscillation of the resistance ofthe MTJ due to the magnetic precession in the free magnetic layer, thusgenerating an oscillation signal. The frequency and amplitude of thegenerated oscillation signal can be used to controlling the sensingcurrent across the MTJ.

The above and other features, and exemplary implementations andapplications, are described in greater detail in drawings, thedescription and the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of a magnetic tunnel junction (MTJ) in a2-terminal circuit configuration.

FIGS. 2A and 2B show examples of magnetic tunnel junction (MTJ) circuitsin a 3-terminal circuit configuration implementing a spin Hall effectmetal layer for providing a spin-polarized current into the freemagnetization layer of the MTJ.

FIGS. 3A and 3B illustrate operation of a spin Hall effect metal layerfor providing a spin-polarized current into the free magnetizationlayer, wherein the flowing directions of the in-plane charge currentJ_(c) (or J_(e)) and out-of-plane spin-polarized current J_(s) and thedirection of the injected spin σ are shown.

FIG. 4 shows an example of a 3-terminal MTJ circuit having a currentsource coupled to the spin Hall effect metal layer and a voltage sourcecoupled across the MTJ.

FIG. 5A shows an example of a schematic perspective-view diagramillustrating a three terminal ST-MRAM device cell that employs the spinHall effect (SHE) and a gate voltage across the MTJ for a writingoperation where the ST-MRAM cell is comprised of a magnetic tunneljunction having in-plane magnetic layers and a non-magnetic strip with astrong SHE and the non-magnetic strip is located on the bottom of theSTT-MRAM device structure.

FIG. 5B shows another example of a three terminal ST-MRAM device cellthat employs the spin Hall effect (SHE) and a gate voltage across theMTJ for a writing operation, where the magnetic tunnel junction hasin-plane magnetic layers and the non-magnetic strip with the strong SHEis located on the top of the STT-MRAM device structure.

FIG. 6A shows an example of a three terminal ST-MRAM device cell thatemploys the spin Hall effect (SHE) and a gate voltage across the MTJ fora writing operation, where the equilibrium positions of the magneticmoments of the FL and PL are perpendicular to the film plane.

FIG. 6B shows an example of a three terminal ST-MRAM device cell thatemploys the spin Hall effect (SHE) and a gate voltage across the MTJ fora writing operation, where the equilibrium positions of the magneticmoments of the FL and PL are perpendicular to the film plane, and anadditional in-plane magnetized ferromagnetic material layer is providedin the MTJ stack to produce an in-plane magnetic bias field for defininga definite switching direction for the perpendicular magnetization ofthe free magnetic layer. This in-plane magnetized ferromagnetic materiallayer in the MTJ stack (e.g., between the first electrical terminal andthe spin Hall effect metal layer as shown) eliminates a separatemagnetic mechanism to produce the magnetic bias field at the freemagnetic layer. A non-magnetic spacer layer can be provided to be incontact with the pinned magnetic layer, and the magnetic layer is incontact with the non-magnetic spacer layer and configured to have amagnetization direction in the magnetic layer to produce the biasmagnetic field in the free magnetic layer.

FIG. 7A shows an example 3-terminal MTJ device for demonstrating thecapability of the effect of voltage control of the magnetic anisotropy(VCMA) to modulate the spin Hall torque switching of the FL of the MTJ.

FIGS. 7B and 7C illustrate operation of a bias voltage, V_(MTJ), appliedacross the tunnel junction terminals in a 3-terminal SHE device tosubstantially alter the current I_(Ta) required through the spin Halllayer to effect either parallel to anti-parallel (P to AP) switching(FIG. 7A) or anti-parallel to parallel (AP to P) switching (FIG. 7B),where the shaded areas indicate the current range in which the ON state(V_(MTJ)=−400 mV) and the OFF state (V_(MTJ)=0 mV) alter the switchingprobability from 100% to zero.

FIG. 7D illustrates gated spin Hall torque switching under a series of10 us pulses, where R_(MTJ) is the resistance of the MTJ (data state).To achieve the gated switching to the high resistance state, V_(MTJ) isswitched between 0 mV and −400 mV, while the spin Hall current I_(Ta) isswitched between 0 mA and −0.55 mA. To achieve the gate switching to thelow resistance state, V_(MTJ) is switched between 0 mV and −400 mV,while the spin Hall current I_(Ta) is switched between 0 mA and 0.35 mA.Switching does not occur unless the V_(MTJ)=−400 mV pulse is applied.

FIG. 8 provides an example of a cross-point memory architecture enabledby gated spin Hall torque switching for an array of 3-terminal memorycells based on coupling between MTJ and the spin Hall effect metal layerwhere transistor switches are shared and coupled to the three terminalsof MTJ cells.

FIGS. 9A and 9B illustrates examples of transistor switch operationstatus for bias configurations that can be employed for the writing andreading operations in the gated spin Hall torque cross-point memoryarchitecture in FIG. 8.

FIG. 10 illustrates an example of an oscillation circuit that can beused to excite magnetic oscillation in a 3-terminal SHE/VCMA device andoutput the associated microwave power to achieve spin torquenano-oscillator performance. The black arrows denote the currentdistribution inside the three terminal SHE device and the spin Hallcurrent I_(Ta) and the tunnel junction bias current I_(MTJ) come fromcurrent source 1 and current source 2, respectively.

FIG. 11 shows the microwave spectra produced by a spin Hall torqueexcited FL in a magnetic tunnel junction when the MTJ bias current isI_(MTJ)=60 μA, and the spin Hall current in the spin Hall metal (Ta)strip is varied between I_(Ta)=−0.8 mA and +0.8 mA. The spectra underdifferent currents are shifted vertically for the ease of comparison.The power spectrum density (PSD) is a measure of the output microwavepower of the device.

FIG. 12A shows the integrated output microwave power of a SHE drivenspin torque nano-oscillator as indicated schematically in FIG. 10. Thered triangles represent the microwave power versus the applied MTJcurrent. The blue circles represent the microwave power normalized byI_(MTJ) ² and the magnetoresistance of the corresponding current.

FIG. 12B shows the center oscillation frequency of a SHE excited andVCMA tuned spin torque nano-oscillator in FIG. 10 as the function of theapplied MTJ current.

FIG. 13 shows an example of a 3-terminal MTJ device having a thintransition metal layer between the free magnetic layer and the SHE metallayer for enhancing the perpendicular magnetic anisotropy in the freemagnetic layer.

FIGS. 14A and 14B show two examples of 3-terminal MTJ devices having athin magnetic insulator layer between the free magnetic layer and theSHE metal layer for reducing leaking of the charge current in the SHEmetal layer into the electrically conductive free magnetic layer.

DETAILED DESCRIPTION

FIG. 2A shows an example of a 3-terminal MTJ device having a spin Halleffect (SHE) metal layer coupled to the free magnetic layer of the MTJjunction. The layers in the MTJ and the SHE metal layer, e.g., selectionof the materials and dimensions, are configured to provide a desiredinterfacial electronic coupling between the free magnetic layer and theSHE metal layer to generate a large flow of spin-polarized electrons orcharged particles in the SHE metal layer under a given charge currentinjected into the SHE metal layer and to provide efficient injection ofthe generated spin-polarized electrons or charged particles into thefree magnetic layer of the MTJ. Each of the free magnetic layer or thepinned magnetic layer can be a single layer of a suitable magneticmaterial or a composite layer with two or more layers of differentmaterials. The free magnetic layer and the pinned magnetic layer can beelectrically conducting while the barrier layer between them iselectrically insulating and sufficiently thin to allow for electrons topass through via tunneling. The spin Hall effect metal layer can beadjacent to the free magnetic layer or in direct contact with the freemagnetic layer to allow the spin-polarized current generated via a spinHall effect under the charge current to enter the free magnetic layer.

The 3 terminals in the MTJ device in FIG. 2A can be used to implementtwo independent control mechanisms that are not possible in the2-terminal MTJ device in FIG. 1. As illustrated, the first controlmechanism is via applying a gate voltage across the MTJ junction withthe first terminal so that the electric field at the free magnetic layercaused by the applied gate voltage can modify the magnetization of thefree magnetic layer including its perpendicular magnetization thataffects the threshold value of a spin-polarized current that can switchthe magnetization of the free magnetic layer via spin torque transferfrom a spin-polarized current that is injected into the free magneticlayer. The second, independent control mechanism uses second and thirdelectrical terminals at two contact locations of the SHE metal layer ontwo opposite sides of the area in contact with the MTJ of the SHE metallayer to supply the charge current in the SHE metal layer to produce thespin-polarized electrons or charged particles based on the spin Halleffect.

In principle, the layers of the MTJ and the SHE metal layer can beconfigured to allow either one of the gate voltage across the MTJ or thecharge current in the SHE metal layer to independently cause switchingof the magnetization of the free magnetic layer. However, in thedisclosed 3-terminal MTJ devices in this document, the gate voltageacross the MTJ is controlled to be less than the threshold voltage thatis sufficient to independently cause a significant current tunnelingthrough the barrier layer of the MTJ to trigger the switching, andsimilarly, the charge current in the SHE metal layer is controlled to beless than the threshold charge current that is sufficient toindependently cause a significant amount of the spin-polarized chargesto enter the free layer to trigger the switching. Notably, the disclosed3-terminal MTJ devices and techniques in this document use the combinedoperation of both the gate voltage across the MTJ and the charge currentin the SHE metal layer to collectively trigger the switching in the freemagnetic layer. In FIG. 2A, a 3-terminal control circuit is coupled tothe first, second and third electrical terminals to achieve the abovedesired control operations.

Specifically, the 3-terminal control circuit is operated as thefollowing. The gate voltage is applied between a first electricalterminal in contact with the pinned magnetic layer and the spin Halleffect metal layer to modify the perpendicular magnetic anisotropy ofthe free magnetic layer, without allowing the gate voltage alone tocause switching of the magnetization direction of the free magneticlayer; and the charge current is applied between two electricalterminals in the spin Hall effect metal layer to induce a spin-polarizedcurrent into the free magnetic layer without switching of themagnetization of the free magnetic layer. The application of the gatevoltage and the application of the charge current are synchronized inorder to switch the magnetization of the free magnetic layer.

FIG. 2B shows an example where the 3-terminal control circuit in FIG. 2Ais implemented by a MTJ circuit and a SHE circuit. The MTJ circuit iscoupled between the first and the third terminals to apply a desiredvoltage across the MTJ without switching the magnetization of the freemagnetic layer. The SHE circuit is coupled between the second and thethird electrical terminals to supply the charge current in the SHE metallayer. A control circuit is further coupled to the MTJ circuit and theSHE circuit to control the operations of the MTJ and the SHE circuits,e.g., controlling the voltage amplitude or direction across the MTJ, thecurrent amplitude or direction of the charge current in the SHE metallayer, and synchronizing the voltage and the charge current in time forswitching the magnetization of the free magnetic layer.

The 3-terminal MTJ devices disclosed in FIGS. 2A, 2B and other parts ofthis document can be implemented to provide circuit configurations andoperational features that are difficult to achieve in 2-terminal MTJdevices and to achieve certain advantages in applications. For example,the charge current applied to the spin Hall effect metal layer via twoelectrical terminals at two contact locations of the spin Hall effectmetal layer is used to inject a spin-polarized current into the freemagnetic layer of the MTJ for effectuating a spin torque transfer intothe free magnetic layer eliminates the need to apply a large currentacross the MTJ for effectuating sufficient spin torque transfer into thefree magnetic layer for switching the magnetization of the free magneticlayer as in the 2-terminal MTJ device. This can be advantageous becausethere are detrimental aspects to effecting the magnetic reorientation ofthe free magnetic layer (FL) with a current pulse that passes throughthe tunnel barrier layer for the memory cell application. For example,the high current pulse required to tunnel through the MTJ junction forthe switching operation can result in degradation of the electricalintegrity of the insulator barrier layer in the MTJ. In a 2-terminal MTJdevice, the design of the FL can be made to reduce the required writecurrent pulse amplitude for the switching operation. However, since thereading operation and the writing operation in this 2-terminal MTJdevice are effectuated via the same two terminals of the MTJ, theelectrical bias required to provide a large enough signal for a fastread of the memory cell can produce a tunneling current through the MTJthat is lower but close to the designed threshold current for theswitching operation of the MTJ. This condition can result in a“write-upon-read” error where the MTJ is inadvertently switched during aread operation due to electrical noise that momentarily adds a smallamount of additional current to the read current. The rate of this“write-upon-read” error increases as the difference between the currenttunneling through the MTJ during a read operation and the STT thresholdcurrent for switching the MTJ becomes smaller. As such, various2-terminal MTJ devices face a conflict between the need to reduce theamplitude of the tunneling current for switching the MTJ and the needfor fast read associated with using a sufficiently large read current tocomplete the measurement of the MTJ resistance for reading the storedbit in a short time. Different from the 2-terminal MTJ devices, the3-terminal MTJ devices in this document are configured to provide twoseparate and independent controls over the voltage across the MTJ toeliminate the above dilemma in the 2-terminal MTJ devices and canachieve a low tunneling current across the MTJ during a write operationwhile still being able to achieve a fast reading operation without beingsubject to the “write-upon-read” error in the 2-terminal MTJ devices. Toeffectuate a switching in the 3-terminal MTJ devices disclosed in thisdocument, the two separate controls are synchronized in order to switchthe magnetization of the free magnetic layer.

For a large array of 3-terminal MTJ cells in various circuits, thecolumn and row driving circuits for the array of 3-terminal MTJ cellscan be designed to reduce the overall circuit size by sharing circuitelements. As described in greater detail in the examples below, across-point memory architecture can be implemented based on the gatedspin Hall torque switching to provide sharing of transistor switches inthe 3-terminal MTJ cells, thus improving the overall compactness ofcircuits using large arrays of 3-terminal MTJ cells.

In another aspect, the availability of three terminals as input/outputports for a 3-terminal MTJ device disclosed in this document can be usedto implement various logic operations. In contrast, with only twoterminals available, the 2-terminal MTJ devices tend to be difficult, orinfeasible in some cases, in building circuits for various binary logicapplications based on the spin-torque switching operations.

In yet another aspect, the 3-terminal MTJs in combination with spintransfer torque disclosed in this document can be configured to employ amagnetic configuration such that the free magnetic layer has only onestable magnetic state but can be excited into magnetic precession aboutthis equilibrium state at microwave or RF frequencies by theanti-damping torque generated by a steady spin-polarized direct currentthat impinges on the free magnetic layer. The frequency of oscillationis determined by the total time-averaged effective magnetic fieldexperienced by the free magnetic layer, and this can vary with theamplitude of the magnetic precession, which in turn depends on theamplitude of the bias current. The time varying magnetoresistance of theMTJ due to the precession of the free magnetic layer provides amicrowave output signal. Thus spin transfer torque can be employed in aMTJ to produce a spin-torque nano-oscillator (STNO) that has potentialapplication in on-chip communication and signal processing applications.In STNO devices based on 2-terminal MTJ devices, the amplitude of theoscillator cannot be electrically varied independently of its frequency,due to the 2-terminal character of the MJT.

Specific implementations and examples of the present 3-terminal MTJdevices and applications are provided below.

The giant spin Hall effect in various heavy (high atomic number) metals,such as Pt, Ta, W, Hf, and others, provides the foundation for the new3-terminal MTJ devices in this document. The spin Hall effect in certainmetals with large atomic numbers is illustrated in FIGS. 3A and 3B. FIG.3A shows a spin Hall effect metal layer is in direct contact with a freemagnetic layer of an MTJ for receiving an in-plane charge current J_(c)(or J_(e)) and for producing a spin-polarized current J_(s) into thefree magnetization layer. The flowing directions of the in-plane chargecurrent J_(c) (or J_(e)) and out-of-plane spin-polarized current J_(s)and the direction of the injected spin are shown. FIG. 3B furtherillustrates that the spin Hall effect separates two spin states in thecharge current in opposite directions that are perpendicular to thein-plane charge current J_(c) (or J_(e)). Hence, by controlling thecurrent direction of the in-plane charge current J_(c) (or J_(e)) in theSHE metal layer, one of the two spin states can be selected as thespin-polarized current J_(s) that is injected into the freemagnetization layer.

FIG. 3B further shows that, the orientation of the injected spins in thespin-polarized current J is determined by a relationship between thecharge current J_(c) (or J_(e)), the direction of the injected spinmoments {right arrow over (σ)} (not the angular momenta) and the chargecurrent J_(c): {right arrow over (J)}∝θ_(SH){right arrow over(σ)}×{right arrow over (J)}_(c), where θ_(SH) is the spin Hall angle andis a parameter specific to each material and quantifies the magnitude ofthe SHE effect in each material.

In the spin Hall effect, an electrical current flowing through a heavymetal thin film layer creates a transverse spin current due to spindependent deflection of electrons in the directions perpendicular to thedirection of current flow. Electrons of opposite spin angular momentumare deflected in opposite directions as illustrated in FIGS. 3A and 3B.In layers of high resistivity beta-Ta, for example, the spin Hall effectis particularly strong with the transverse spin current density being ashigh as 0.15 of the longitudinal electrical current density. This spincurrent can be utilized to exert torque on the magnetization of anadjacent magnetic film, and thus enables a 3-terminal magnetic circuitor device for reversing the magnetic orientation of the FL of a magnetictunnel junction that is formed on top of a spin Hall layer, as alsoillustrated in FIGS. 2A and 2B.

FIG. 4 shows an example of a 3-terminal MTJ circuit that includes avoltage source coupled between the first and third electrical terminalsacross the MTJ and a current source coupled between the second and thirdelectrical terminals to the spin Hall effect metal layer. The FL and PLlayers in this example are shown to be parallel to the planes of thelayers as in-plane magnetization that is perpendicular to the directionof the in-plane charge current J_(c) (or J_(e)) in the SHE metal layer.

The present 3-terminal MTJ devices operate to effectuate switching ofthe magnetization in the free magnetic layer by simultaneously applyingthe gate voltage across the MTJ junction and the charge current in theSHE metal layer. This aspect of the 3-terminal MTJ devices is based onvoltage-controlled magnetic anisotropy (VCMA), in which an electricfield alters a ferromagnetic film's perpendicular anisotropy by changingthe electronic structure at a ferromagnet/oxide interface. VCMA has beenshown to enable strong tuning of the coercive magnetic field of the FLin a MTJ and direct toggle switching of the FL by voltage pulses appliedacross the MTJ. A significant aspect of VCMA is that it offers thepotential of effecting the switching of a FL with little or no currentflow through the MTJ, which could lower the energy cost of the MRAMwrite operation by minimizing Ohmic loss.

Considering the example in FIG. 4, the in-plane charge current J_(e) inthe SHE metal layer is set to produce the spin-polarized J_(s) that isperpendicular to the in-plane charge current J_(e) in the SHE metallayer. When the SHE metal layer is sufficiently thin in the transversedirection, the spin-polarized J_(s) is injected into the freemagnetization layer without significantly losing the injected spinmoments {right arrow over (σ)} due to the spin relaxation caused bypropagation of the electrons or charged particles. The magnitude of thein-plane charge current J_(e) in the SHE metal layer is controlled to besufficiently small so that the spin-polarized current J_(s) that hasentered the free magnetization layer is significantly smaller than thethreshold current for the spin-polarized current to cause switching ofthe magnetization of the free magnetic layer. The gate voltage acrossthe MTJ junction, however, is applied to alter the perpendicularanisotropy by changing the electronic structure at the ferromagnet/oxideinterface due to the voltage-controlled magnetic anisotropy (VCMA) tolower the threshold current required for the spin-polarized current tocause switching of the magnetization of the free magnetic layer to alevel that the spin-polarized current J_(s) that has entered the freemagnetization layer is at or above the newly reduced threshold currentfor switching the MTJ. Under this condition of simultaneous applicationof the charge current and the gate voltage, the magnetization of thefree magnetic layer is switched.

FIG. 5A shows an example of a schematic perspective-view diagramillustrating a three terminal ST-MRAM device cell that employs the spinHall effect (SHE) and a gate voltage across the MTJ for a writingoperation where the ST-MRAM cell is comprised of a magnetic tunneljunction having in-plane magnetic layers and a non-magnetic strip with astrong SHE and the non-magnetic strip is located on the bottom of theSTT-MRAM device structure. FIG. 5B shows another example of a threeterminal ST-MRAM device cell that employs the spin Hall effect (SHE) anda gate voltage across the MTJ for a writing operation, where themagnetic tunnel junction has in-plane magnetic layers and thenon-magnetic strip with the strong SHE is located on the top of theSTT-MRAM device structure.

The SHE and VCMA can also be combined to yield gate controlled SHEswitching of the FL in a MTJ in the case where the magnetic moments m ₁and m ₂ of the free layer and reference layer of the MTJ are orientedperpendicular to the plane of the films. In this configuration, theinjected spins from the SHE {right arrow over (σ)} are still along +/−x-axis in the plane of MTJ layers while the equilibrium position for m ₁is aligned along the +/− z axis perpendicular to the MTJ layers. So, thedirection of m ₁ and that of {right arrow over (σ)} are perpendicular toeach other. In this situation the effect of the spin torque from thespin current generated by the SHE can be described using an effectivemagnetic field H_(ST).

FIG. 6A shows an example of a three terminal ST-MRAM device cell thatemploys the spin Hall effect (SHE) and a gate voltage across the MTJ fora writing operation, where the equilibrium positions of the magneticmoments of the FL and PL are perpendicular to the film plane.

FIG. 6B shows an example of a three terminal ST-MRAM device cell thatemploys the spin Hall effect (SHE) and a gate voltage across the MTJ fora writing operation, where the equilibrium positions of the magneticmoments of the FL and PL are perpendicular to the film plane, and anadditional in-plane magnetized ferromagnetic material layer is providedin the MTJ stack to produce an in-plane magnetic bias field for defininga definite switching direction for the perpendicular magnetization ofthe free magnetic layer.

Embodiments of the above new 3-terminal MTJ device configuration can beused to solve the reliability challenges that presently limitapplications based on various two-terminal MTJ devices while also givingimproved output signals. This new 3-terminal MTJ device configurationcan also provide the added advantage of a separation between thelow-impedance switching (write) process and high-impedance sensing(read) process in MTJ memory devices. More specifically, the devices andmethods discloses here combine the spin Hall effect (SHE) with thevoltage control of the magnetic anisotropy (VCMA) of nanoscale magneticelements to enable the electrically gated switching of the magneticorientation of a bi-stable magnetic element in a magnetic tunneljunction, and the electrical tuning of the oscillation frequency andoutput power of a spin torque nano-oscillator (STNO). This 3-terminalMTJ design enables more efficient and effective designs of magneticrandom access memory circuits and of high performance non-volatile logiccircuits, and a new 3-terminal approach to STNO's that providesseparate, independent control of the oscillation microwave amplitude andfrequency.

In implementations, the materials of the MTJ layers suitable for thedisclosed 3-terminal MTJ devices are selected to form a magnetic tunneljunction that exhibits a strong voltage-controlled magnetic anisotropy(VCMA) effect, with its free layer located adjacent to a non-magneticmetallic strip composed of a material having a strong spin Hall effect(SHE) that can carry current flowing in the film plane. In someimplementations, the magnetic tunnel junction is comprised of twoferromagnetic thin film elements separated by a thin, less than 2.0 nmthick, insulating layer, typically MgO or some other insulator material,that serves as a tunnel barrier through which electrons can tunnel by aquantum mechanical process. One of the ferromagnetic elements, thepinned layer (PL), which may or may not consist of multiple layers ofthin film material, has a fixed magnetization direction, and the otherferromagnetic layer, the free layer (FL), which may or may not consistof multiple layers of thin film material, is free to rotate under theinfluence of a strong enough spin current or applied magnetic field.Depending on whether the magnetization of the FL is aligned, as resultof the action of a spin current, more or less parallel or anti-parallelto the magnetization direction of the PL, the resistance of the MTJ iseither in its low resistance state (parallel) or high resistance state(anti-parallel). The MTJ is fabricated to have a magnetoresistancechange of 10% or more.

The material composition of the insulating layer and the adjacent FLsurface are also chosen such that the electronic interface between thetwo results in a substantial interfacial magnetic anisotropy energy thatalters the perpendicular magnetic anisotropy of the FL. Appropriatecombinations of material include, but are not limited to, MgO for theinsulating layer and for the interfacial surface layer of the FL, Co,Fe, and alloys with Co and/or Fe components. The interfacial electronicstructure is such that an electric field that is produced by theapplication of a voltage bias across the insulator layer cansubstantially modify the interfacial magnetic anisotropy energy,resulting in a voltage controlled magnetic anisotropy (VCMA) of the FL.In some MTJ device implementations, changes in the interfacialmagnetization energy per unit electric field of 25 μJ/m² (V/nm)⁻¹ orgreater can effectuate the necessary change in magnetic anisotropy.

In making the 3-terminal MTJs, the magnetic tunnel junction isfabricated such that its free layer is adjacent to and in goodelectrical contact with a thin film strip composed of a material thathas a high spin Hall angle, e.g., greater than 0.05, as a spin Halleffect (SHE) metal layer to generated a spin-polarized current. Forexample, in implementations, this SHE metal layer can have a thicknessthat is less than or no more than approximately five times its spindiffusion length to maintain sufficient spin population in a particularspin state in the generated spin-polarized current at the interface withthe free magnetic layer of the MTJ. An electrical current passingthrough this SHE metal thin film strip can provide, via the spin Halleffect, a transverse spin current that will exert spin torque on the MTJFL that is sufficient to either efficiently reverse its magneticorientation, depending on the direction of current flow through the spinHall layer, or alternatively to excite it into persistent microwaveoscillation, while a bias voltage across the MTJ is employed to modifythe magnetic anisotropy and/or coercive field of the FL via the VCMAeffect. This combination achieves new spin-transfer-torque devicefunctionalities: gate-voltage-modulated spin torque switching andgate-voltage-modulated spin torque oscillation. The former makespossible energy-efficient and gate controlled switching for non-volatiledigital logic application, and more energy-efficient and improvedarchitectures for non-volatile digital memory applications, including asimple approach for implementing magnetic memory circuits with amaximum-density cross-point geometry that does not require a controltransistor for every MTJ. The latter provides separate, independentcontrol of the microwave oscillation amplitude and frequency of a spintorque nano-oscillator.

Referring to the 3-terminal MTJ device examples in FIGS. 2A, 2B, 4, 5A,5B, 6A and 6B, a 3-terminal MTJ device can be configured to include amagnetic tunneling junction (MTJ) that includes (1) a pinned magneticlayer having a fixed magnetization direction, (2) a free magnetic layerhaving a magnetization direction that is changeable, and (3) anon-magnetic junction layer between the magnetic free layer and thepinned magnetic layer and formed of an insulator material sufficientlythin to allow tunneling of electrons between the magnetic free layer andthe pinned magnetic layer, and a spin Hall effect metal layer that isnonmagnetic and includes a metal exhibiting a large spin Hall effect toreact to a charge current directed into the spin Hall effect metal layerto produce a spin-polarized current that is perpendicular to the chargecurrent, the spin Hall effect metal layer being parallel to and incontact with the free magnetic layer to direct the spin-polarizedcurrent generated in the spin Hall effect metal layer into the freemagnetic layer. The 3-terminal MTJ device also includes a firstelectrical terminal in electrical contact with the MTJ from a sidehaving the pinned magnetic layer to receive a gate voltage that modifiesa current threshold of a spin-polarized current flowing across the MTJfor switching the magnetization of the free magnetic layer, and secondand third electrical terminals in electrical contact with two contactlocations of the spin Hall effect metal layer on two opposite sides ofthe free magnetic layer to supply the charge current in the spin Halleffect metal layer. A control circuit is coupled to the first, secondand third electrical terminals to supply (1) the charge current via thesecond and third electrical terminals in the spin Hall effect metallayer and (2) the gate voltage across the MTJ causing a small currenttunneling across the MTJ that is insufficient to switch themagnetization of the free magnetic layer without collaboration of thespin-polarized current flowing across the free magnetic layer caused bythe charge current.

For memory applications, the control circuit in the 3-terminal MTJdevice can be specifically configured to be operable in a writing modeto simultaneously apply the charge current in the spin Hall effect metallayer and the gate voltage across the MTJ to set or switch themagnetization direction of the free magnetic layer to a desireddirection for representing a stored bit, and, in a read mode, thecontrol circuit is operable to apply a read voltage to the firstelectrical terminal to supply a read current tunneling across the MTJbetween the first electrical terminal and the spin Hall effect metallayer, without switching the magnetization direction of the freemagnetic layer, to sense the magnetization direction of the freemagnetic layer that represents the stored bit in the MTJ.

Referring to FIGS. 5A and 5B, the SHE is employed as the writingmechanism and a magnetic tunnel junction (MTJ) is employed both to applythe gate voltage that modulates the magnetic orientation of the freelayer (FL) during the application of the spin torque effect by the SHEgenerated spin current, and to sense the magnetic orientation of thebistable free layer relative to that of the fixed reference layer (RL).The MTJ can be a pillar-shaped magnetic device with the lateraldimension generally in the sub-micron or nanometer range. The freeferromagnetic layer with the magnetic moment is made of softferromagnetic material with small to medium coercive field. The pinnedferromagnetic layer with magnetic moment is made of soft or hardferromagnetic material with large coercive field or pinned by additionalantiferromagnetic layers. Typical thickness for the free and pinnedmagnetic layers range from less than one nanometer to a few tens ofnanometers. The FL and the PL are separated by a crystalline insultingspacer layer, less than 2 nm in thickness, such as MgO or boron dopedMgO (Mg(B)O) or any other crystalline or amorphous insulator layer thatgenerates an interfacial magnetic anisotropy energy density per unitarea of contact with the surface of the ferromagnetic free layer thatsubstantially affects the overall magnetic anisotropy of the FL. Thismagnetic anisotropy energy density can be substantially modified byelectric fields applied across the insulator-FL interface. Examples ofsuitable materials for the magnetic layer may include (but are notlimited to) Fe, Co, Ni, alloys of these elements, such asNi_(1−x)Fe_(x), alloys of these elements with non-magnetic material,such as Fe_(1−x)Pt_(x) and Co_(x)Fe_(y)B_(1−(x+y)), and ferromagneticmultilayers made from those materials, such as (Co/Ni)_(n), (Co/Pt)_(n),and (Co/Pd)_(n) where n represents the repeat number of the multilayer.that the materials for the MTJ structures are selected so that there bea substantial interfacial magnetic anisotropy energy density per unitarea of contact between the surface of the ferromagnetic free layer thatis in contact with the insulator layer and that this anisotropy varysignificantly with the voltage that is applied between a ferromagneticreference layer on one side of the insulator and the free layer on theother side. Variation is the strength of this applied voltage varies theelectric field at the insulator-free layer interface and hence modifiesthe interfacial magnetic anisotropy experienced by the FL.

In contact with the FL of the magnetic tunnel junction is a non-magneticthin-film strip made of one of a variety of materials that exhibit astrong spin Hall effect (SHE). Examples of suitable materials for thislayer include high resistivity Ta (beta-Ta), W (beta-W), Hf and Irlayers. Other suitable materials for the SHE layer include (but are notlimited to) Pt, Pd, Nb, Mo, Ru, Re, Os, Ir, Au, Tl, Pb, Bi as well asthe alloys based upon those transition metals such as Cu_(1−x)Bi_(x),Ag_(1−x)Bi_(x), Cu_(1−x)Ir_(x), Ag_(1−x)Ir_(x), Cu_(1−x)W_(x),Ag_(1−x)W_(x), Cu_(1−x)Ta_(x), Ag_(1−x)Ta_(x), Hf_(x)Ir_(y) and highresistivity intermetallic compounds that incorporate one or moreelements with high atomic number, such as compounds with the A15 crystalstructure such as Ta₃Al, Nb₃Sn, W₃Ge, Ir₃Hf and other compounds such asTaN, WN and NbN. The non-magnetic SHE strip is patterned into ananometer scale or micrometer scale width and has a thickness that isless than or approximately equal to five times its spin diffusionlength.

In the examples in FIGS. 5A and 5B, three terminals are formed whereelectrical connections are made to the device. One terminal is on thepillar, close to the PL of the MTJ, and the other two terminals are thetwo ends of the non-magnetic strip. Writing current is applied betweenthe two terminals on the non-magnetic strip while a bias voltage isapplied between the terminal on the pillar and either one of the twoterminals on the non-magnetic strip to effect the gating of theswitching of the FL magnetization or alternatively to modulate theoscillator frequency in a spin torque nano-oscillation implementation.To read the binary state of the device for a logic gate or memory deviceimplementation, a bias current is applied between the terminal on thepillar and either one of the two terminals on the non-magnetic strip.

In FIGS. 5A and 5B, the FL of the MTJ can be either at the bottom of thepillar, as shown in FIG. 5A, or on the top of the pillar as illustratedin FIG. 5B. In either case the non-magnetic strip with the strong SHE isalways adjacent to the FL. When the FL is at the bottom, thenon-magnetic strip is also at the bottom of the device, next to thesubstrate. When the FL is on the top, the PL is placed on the substrateside of the tunnel barrier, the FL is above the tunnel barrier, and thenon-magnetic strip is located on the top of the device.

When the FL and RL are polarized in plane in a 3-terminal MTJ device,and with their in-plane magnetization direction perpendicular to thecurrent direction mentioned above (i.e. along +/− x axis direction), m ₁is collinear (either parallel or anti-parallel) with the injected spinsfrom the SHE {right arrow over (σ)}. In this case the injected spinsfrom the SHE act as an effective magnetic damping which depending uponthe orientation of the spin can be of either sign, i.e. either positiveor negative damping. Under this configuration, the SHE induced switchingworks in the same way as the conventional spin torque induced switching.Conventional spin torque switching employs a pair of ferromagneticlayers separated by a non-magnetic spacer layer, with one ferromagneticlayer being the fixed, polarizer layer, and the other ferromagneticlayer being the free layer whose magnetic moment orientation can beswitched by the transfer of spin torque from the polarized current. Onedifference is that, the spin current in the spin Hall effect device isgenerated using a non-magnetic material instead of a ferromagneticpolarizer layer. When m ₁ is parallel with {right arrow over (σ)}, thespin current will make the current magnetization orientation morestable, and will not cause switching. Otherwise, when m ₁ isantiparallel with {right arrow over (σ)}, if the spin current is largeenough, the magnetization of FL will be switched. Therefore, currentswith opposite signs inject spins with an opposite orientation into theFL, and those opposite orientations will result in different preferableorientations of the FL magnetization, so a reversible deterministicswitching can be realized by determining the direction of the electricalcurrent through the SHE generating layer.

The result of this combining the spin torque exerted by the spin Halleffect with the voltage-controlled magnetic anisotropy (VCMA) effect isthat, in the absence of thermal fluctuations, the critical or thresholdcurrent density required to flow through the lateral spin Hall layer tocause spin torque switching of an in-plane polarized magnetic free layerby the spin Hall effect depends on the effective perpendiculardemagnetization field H_(demag) ^(eff) of the free layer as

$\begin{matrix}{{J_{c\; 0}} \approx {\frac{2{eM}_{S}t_{free}\alpha}{{\hslash\theta}_{SH}}{\left( {H_{c} + {0.5H_{demag}^{eff}}} \right).}}} & (1)\end{matrix}$

As result of the VCMA effect H_(demag) ^(eff) is variable as a functionof the voltage V_(MTJ) applied across the MTJ:H _(demag) ^(eff)=4πM _(S)−2K _(u)(V _(MTJ))/M _(S).  (2)

Here e is the electron charge, M_(S) is the saturation magnetization ofthe CoFeB free layer, t_(free) is its thickness and α the value of itsGilbert damping, H_(c) is its within-plane magnetic anisotropy field,and K_(u) (V_(MTJ)) is the voltage-dependent perpendicular anisotropyenergy coefficient of the free layer. Thus as indicated by Equations (1)and (2), the critical current density that is required to flow throughthe SHE layer to effect the switching of the FL of the MTJ can bemodulated by applying a gating voltage to the MTJ. In an implementationof this device by the current inventors d(H_(demag)^(eff))/dV_(MTJ)=730±90 Oe/V was achieved, corresponding to a change indemagnetization energy per unit electric field|d(K_(u)t)/dE|=[M_(S)t_(free)t_(MgO)/2]d(H_(demag) ^(eff))/dV=70 μJ/m²(V/nm)⁻¹. Values for the modulation of the magnetic anisotropy by theapplied electric field that are as much as a factor of 3 lower than 70μJ/m² (V/nm)⁻¹, and values that are higher than this can also beeffective in this invention.

For digital logic and gated memory embodiments of this invention theVCMA must be capable of changing the probability of SHE spin torqueswitching of the MTJ free layer fully between 0% and 100% for a givenlevel of applied current through the spin Hall layer. For long pulselengths, e.g., greater than 10 ns, and at room temperature and above,thermal activation of the FL can contribute substantially to itsreversal. The energy barrier E that the thermal activation energy has toovercome scales directly with the in-plane coercive field H_(c) of thefree layer if the FL is magnetized in plane. Since H_(c) can depend onthe out-of-plane magnetic anisotropy of the FL this means that the gatevoltage can act to modulate the spin Hall torque switching current viaits effects on both the zero-fluctuation critical current density|J_(c0)| and the activation barrier E. However, for most applications,switching will be driven by spin Hall current pulses in a short duration(e.g., less than 10 ns or 20 ns) for which thermal activation provideslittle assistance, although it does result in a probabilisticdistribution of switching current density about |J_(c0)|. Therefore, inthis short pulse regime, the gate voltage can effectively modulate theswitching current density through its influence on |J_(c0)| alone. Forexample, an optimized value of the effective perpendicular magneticanisotropy of the free layer would be H_(demag) ^(eff)≈1 kOe, whiled(H_(demag) ^(eff))/dV_(MTJ)≈700 Oe/V has been established as a typicalvalue of the VCMA effect in, for example, a CoFeB/MgO/CoFeB magnetictunnel junction. Also H_(c) can be readily adjusted so that it is largeenough to maintain the thermal stability of the free layer but H_(c) ismuch less than the perpendicular demagnetization field H_(demag) ^(eff).Using the typical parameter values M_(S)=1100 emu/cm³, t_(free)=1.5 nm,α=0.021, and θ_(SH)=0.15, Eq. (2) yields |J_(c0)|=9.6×10⁶ A/cm² forV_(MTJ)=500 mV and |J_(c0)|=4.5×10⁶ A/cm² for V_(MTJ)=−500 mV. Thisvariation of a factor of two in |J_(c0)| larger than the typical widthof the thermal distribution for the switching current density in spintorque devices, so that the effect of the voltage-controlled anisotropyon J_(c0) is sufficient to achieve full modulation of short-pulse, ≦20nanosecond, spin Hall torque switching of the Fl in optimized spin Hallspin torque devices.

A sample 3-terminal MTJ device was fabricated using a 6 nm thick, 1 μmwide Ta strip as the SHE metal layer and a MTJ stack ofCo₄₀Fe₄₀B₂₀(1.5)/MgO(1.2)/Co₄₀Fe₄₀B₂₀(4) (thicknesses in nanometers) ontop of the Ta SHE metal layer (FIG. 7A. The MTJ stack is shaped to be anapproximately elliptical cross section of 100×350 nm² with the long axisperpendicular to the Ta SHE strip. The sample MTJ device was tested todemonstrate the capability of the VCMA effect to modulate the spin Halltorque switching of the FL of the MTJ by using a long, pulse regime (˜10μs). The test results are shown in FIGS. 7B and 7C, using V_(MTJ)=0 and−400 mV. For both P-to-AP (FIG. 7B) and AP-to-P (FIG. 7C) switchingthere is a window of current amplitude for which the switchingprobability is 100% for V_(MTJ)=−400 mV and 0% for V_(MTJ)=0, so thatV_(MTJ) gates the switching process effectively. This is shown directlyin FIG. 7D; a voltage V_(MTJ)=−400 mV puts the device in the ON statefor switching by the spin Hall torque from the Ta, while V_(MTJ)=0 turnsthe switching OFF. FIG. 7D also demonstrates how this 3-terminal deviceachieves fundamental logic operations by combining spin Hall torqueswitching with the VCMA effect. More complicated logic functions can beobtained by combining more than one spin Hall torque/VCMA device.

To achieve a large spin Hall effect sufficient for efficient switchingof either an in-plane or out-of-plane magnetized magnetic free layerrequires the use of a thin film material comprised of one or moremetallic atomic elements with a high atomic number, and one in whichthere a strong spin-orbit interaction between the conduction electronsand the metallic ions. Materials that are suitable for the disclosed3-terminal MTJ devices include the high atomic number (Z) metallicelements Ta, W, Hf and Ir, all of which, in the appropriate atomicstructural form, have spin Hall angles of greater than 0.08 and in somecases greater than 0.25. Alloys and intermetallic compounds of theseelements and in combination with other high Z elements may also be used.However a metal layer with a high atomic number is not in and of itselfsufficient for effective use as the source of the spin current in thisinvention. In various implementations, the material is selected to haveparticular electronic properties and an optimal crystalline structure,including in relation to the properties and structure of the adjacentferromagnetic layer on which the spin current generated by the spin Halleffect in the first layer acts to effect the magnetic switching or theexcitation of that second, ferromagnetic layer.

First, the electronic properties of the spin Hall metal can beconfigured such that there is a high efficiency in the generation of atransverse spin current density by a longitudinal electronic currentdensity with that conversion efficiency being quantified by what isknown as the spin Hall angle which is defined as the ratio of thetransverse spin conductivity to the longitudinal electronicconductivity, or equivalently the ratio of the generated transverse spincurrent density to applied longitudinal electrical current density. Whena crystalline metal is employed and in the case where the spin Halleffect is intrinsic and arises from the spin-orbit interaction betweenthe conduction electrons and the fixed ionic crystalline latticestructure, which then determines the transverse spin conductivity of thematerial, the electrical conductivity of the metal should be low so thatthe spin Hall angle, or efficiency of the generation of the transversespin current, is high. In the instance, which can also be employed forthis invention, where the spin Hall effect is not intrinsic but isdetermined by the spin-dependent scattering of the conduction electronsby impurities and crystalline defects, that spin-dependent scatteringmust be made strong by choice of the impurities or defects, relative tothe any non-spin dependent scattering of the electrons.

Second, the spin relaxation length within the spin Hall metal is desiredto be short, e.g., less than or equal to 1 nm up to approximately 5 nm.The thickness of the spin Hall layer, in order to optimize conversionefficiency, is no less than approximately one spin relaxation length andno more than approximately five times the spin relaxation length. Thecurrent required to effect the magnetic switching or excitation of theadjacent magnetic layer scales directly with the thickness of the spinHall layer times the spin Hall angle of the material. Therefore tominimize the required switching current a thin spin Hall layer with ahigh spin Hall angle and a short spin diffusion length is optimal.

Third, the electronic structure of the spin Hall material and of theadjacent ferromagnetic material is selected such that a conductionelectron from the spin Hall layer can pass readily across the interfaceinto the ferromagnetic layer if the magnetic moment of the electron isaligned either parallel, or in some cases anti-parallel but usuallyparallel, to the orientation of the magnetization of the ferromagneticlayer and has a low probability of passing into the ferromagnetic layerif the electron's magnetic moment has the opposite orientation relativeto that of the ferromagnetic layer's magnetization. In the case of acrystalline spin Hall material and a crystalline ferromagnetic layer theelectronic band structures of the two materials must be such that theprobability of electron transmission from the spin Hall material acrossthe interface and into either the majority electron sub-band structureor the minority electron sub-band structure of the ferromagnetic layeris much greater in one case than the other. The band structure of thebeta form of Ta, which is generally reported to have tetragonalcrystalline symmetry, is sufficiently different from that of typicalferromagnetic materials, such as FeCo and NiFe alloys as to meet thisrequirement. This is also the case for the beta form of W, which isgenerally reported to have the A15 crystalline symmetry. For Hf, whichcan be found in multiple crystalline forms, including hexagonally closepacked (hcp) and face-centered cubic (fcc) forms, the choice of thecrystalline form relative to that of the composition and crystallineform of the ferromagnetic layer is critical to obtaining a combinationwith a high spin torque efficiency.

Fourth, in implementations where the incident spin current from the spinHall layer excites and then reverses the orientation of theferromagnetic layer by the exertion of an anti-damping spin torque, itis also required that the injection of spins from the magneticallyprocessing ferromagnetic material during this excitation process backinto the spin Hall material is minimized. This injection is known asspin pumping and is generally considered to depend on the probability ofelectronic transmission across the interface per unit area, where thetransmission probability is dependent upon the spin orientation of theelectron relative to that orientation of the magnetization direction ofthe ferromagnet. A high spin pumping rate acts to damp the magneticexcitation of the ferromagnet and hence leads to the undesirablerequirement of a stronger incident spin current density to effect themagnetic switching. This spin pumping process is generally characterizedby a parameter known as the interfacial spin-mixing conductance. Foroptimal performance this spin-mixing conductance should be minimized,well below that found in most conventional combinations of ferromagneticmaterials and high atomic number spin Hall materials. For example theCo—Pt combination has a high spin mixing conductance, as does thecombination of CoFe (or CoFeB) with alpha-W, that is W in the standardbcc crystalline form. However both beta-Ta and beta-W in combinationwith ferromagnetic layers such as CoFe, CoFeB and NiFe alloys exhibit alow spin-mixing conductance, which makes these combinations effectivefor the anti-damping switching embodiment of this invention.

A spin Hall material suitable for implementing the 3-terminal MTJdevices can be selected or designed to have a strong spin orbitinteraction (with a high spin Hall angle and associated high spincurrent density generation efficiency), and a short spin relaxationlength for efficient injection of the spin-polarized electrons or othercharge particles into the FL from the SHE metal layer, (e.g.,approximately 1 to 5 nm). Furthermore the interfacial electronicstructures of the two materials are configured such that the incidentspin current exerts a highly efficient spin torque on the ferromagneticmaterial, this depends on a strong difference in the spin dependentelectron transmission probabilities of the interface. In someimplementation, the 3-terminal MTJ devices can be configured to utilizeanti-damping excitation of the ferromagnetic material to effect theswitching the interfacial electronic properties so that the spin pumpingefficiency, or equivalently that the spin mixing conductance, of theinterface is quite low.

In addition, the insulting spacer layer for the 3-terminal MTJ devicescan range in thickness, e.g., from less than 1 nm to greater than 2 nmin some implementations. The insulting spacer layer can be composed ofpolycrystalline MgO or mixed oxide such as Mg_(x)B_(y)O_(z) of variablecomposition, or any other crystalline or amorphous insulator layer thatresults in a high tunneling magnetoresistance for currents flowingbetween the ferromagnetic reference layer and ferromagnetic free layerplaced on the opposing sides of the insulating layer, and that alsoresults in an interfacial magnetic anisotropy energy density per unitarea of contact with the surface of the ferromagnetic free layer (FL)that substantially affects the overall magnetic anisotropy of that thinFL, and where this magnetic anisotropy energy density can besubstantially modified by electric fields applied across theinsulator-FL interface.

Some examples of the materials for the magnetic free layer may include(but are not limited to) Fe, Co, Ni, alloys of these elements, such asFe_(1−x)Co_(x), Ni_(1−x)Fe_(x), alloys of these elements withnon-magnetic material, such as Fe_(1−x)Pt_(x) andCo_(x)Fe_(y)B_(1−(x+y)), and ferromagnetic multilayers made from thosematerials, such as (Co/Ni)_(n), (Co/Pt)_(n), and (Co/Pd)_(n) where nrepresents the repeat number of the multilayer. Such materials shouldexhibit a substantial interfacial magnetic anisotropy energy density perunit area of contact between the surface of the ferromagnetic free layerthat is in contact with the insulator layer. This interfacial anisotropycan vary significantly with the voltage that can be applied between aferromagnetic reference layer on one side of the insulator and the freelayer on the other side. Variation in the strength of this appliedvoltage changes the electric field at the insulator-free layer interfaceand hence modifies the interfacial magnetic anisotropy experienced bythe FL.

The current that flows through the insulator layer of the magnetictunnel junction during the electrically gated switching operation can bevaried over a wide range by choice of the insulator material and itsthickness. The tunneling resistance of such an insulator layer variesexponentially with its thickness, typically increasing by about oneorder of magnitude for a 0.2 to 0.3 nm increase in thickness, as in thecase of an MgO insulator layer in an MTJ. Thus by using a relativelythick >1.5 nm MgO layer, for example, the tunnel current that flowsthrough the insulator layer due to the voltage bias can be quite lowduring the gated spin Hall switching operation. This can reduce theenergy required for the gate component of the switching operation to thelevel of that required to charge the voltage across the tunnel barrierwhich acts as a capacitor in this case. The voltage required to executethe gated response does vary linearly with insulator layer thickness, sothat a thicker barrier does require a proportionately higher gatevoltage to execute the gated response. Thus the insulator thicknessshould be typically kept to be ≦2 nm in some applications.

Alternatively if the insulator layer is made thin, of the order of 1 nm,then the current that flows through the insulator layer when a voltagebias is applied to modulate the interfacial anisotropy of the free layercan be substantial. Depending on the relative orientation of the FLrelative to the RF, and on the polarity of the voltage bias and hencethe direction of the tunneling electron flow, this current will exert aspin torque on the FL that will either aid or hinder the spin torqueswitching of the FL by a current that is also applied to flow throughthe adjacent spin Hall metal layer. This can add extra flexibility indesigning the device for optimum switching performance and for alsoachieving maximum thermal stability in the absence of gated switchingpulses. The insulator thickness should be thick enough such that thecurrent that flows when a bias voltage that is required to be applied toread the magnetoresistive state of the MTJ during a read operation isnot sufficient to independently effect a switching of the free layer dueto the spin torque exerted by the tunnel junction, without the aid ofany spin torque being generated by a bias current flowing through thespin Hall metal layer.

In addition to providing a new, basic element for high-performancenon-volatile logic circuits, embodiments of the present 3-terminal MTJdesigns enable improved circuit architectures for high-performancemagnetic memory logic technologies. For example, this spin Halltorque/VCMA device can be employed to produce nonvolatile magneticrandom access memory circuits in the maximum-density cross-pointgeometry shown schematically in FIG. 8. A major challenge for thesuccessful implementation of cross-point memories using conventionalspin torque switching with 2-terminal magnetic tunnel junctions is theissue of current flow via sneak paths that leads to unintended switchingevents and increased power consumption during writing processes anddecreased sensitivity during reading. In the circuit shown in FIG. 8,during the writing operation each memory cell can be addressedindividually by applying a gate voltage to the MTJ from above while alsoapplying a current through the SHE microstrip below to generate a spinHall torque.

The device in FIG. 8 includes rows and columns of 3-terminal MTJ memorycells. Rows of spin Hall effect metal stripes are provided and each rowspin Hall effect metal stripe is configured to be in contact with a rowof memory cells as the spin Hall effect metal layer for each memory cellin the row of memory cells and is further coupled to the memory controlcircuit to carry a row charge current as the charge current for eachmemory cell in the row of memory cells. The device in FIG. 8 alsoincludes columns of conductive stripes and each column conductive stripeis configured to be in contact with a column of memory cellsrespectively located in different rows of memory cells and furthercoupled to the memory control circuit to apply a row gate voltage as thegate voltage, or a row read voltage as the read voltage, for each memorycell in the column of memory cells. The memory control circuit includesfirst transistors coupled to the column conductive stripes,respectively, one first transistor per column conductive stripe to applythe row gate voltage or the row read voltage to the first electricalterminals of the memory cells; and second transistors coupled to the rowspin Hall effect metal stripes, respectively, one second transistor perrow spin Hall effect metal stripe to connect to the second electricalterminals to switch on or off the row charge current in the respectiverow spin Hall effect metal stripe as the charge current for each memorycell in a corresponding row of memory cells. In some implementations,the third electrical terminals are grounded. In the example in FIG. 8,this grounding is controlled by third transistors coupled to the rowspin Hall effect metal stripes, respectively, one third transistor perrow spin Hall effect metal stripe to connect between the thirdelectrical terminals of memory cells in a corresponding row of memorycells and an electrical ground.

FIGS. 9A and 9B show examples of operations of the first, second andthird transistors in FIG. 8 during writing and reading operations. Morespecifically as illustrated in FIG. 9A for the writing operation, thefirst transistor at the chosen column and the pair of the second andthird transistors at the two ends of the chosen row are set to be ONwhile all of the other transistors are set to be OFF. V_(switch) ischosen to be positive or negative depending on which final state isdesired for the MTJ. Information is then written into the selected MTJfor the writing operation. MTJs with high impedance can be utilized suchthat R_(MTJ), the impedance of the tunnel junction, is much greater thanR_(Ta), the resistance of the SHE strips. This condition effectivelyblocks all possible sneak paths for the writing current. For the readingoperation shown in FIG. 9B, a parallel reading scheme can be employed toeffectively ameliorate the effect of sneak currents. The secondtransistors for all of the columns and the third transistor at the rightend of the chosen row are set to be ON. All of the other transistors areset to be OFF. Therefore, all of the column lines are set at the sameread voltage level +V. Information is read in a parallel way from allthe MTJs on the same row by measuring the currents flowing in the columnlines.

The overall benefit of the cross-point architecture as illustrated inthe examples in FIGS. 8, 9A and 9B is that whereas some 2-terminalspin-torque MRAM circuits require at least 1 transistor for every bit,the cross-point geometry for the present 3-terminal MTJ circuits can bemade with only 1 transistor for every N bits in an array, therebyincreasing the storage density significantly and reducing the complexityat the interface between the MTJs and the semiconductor (CMOS) circuitelements that provide the write signals and perform the read-out of thestored data.

Referring to FIGS. 6A and 6B where the magnetic layers of the MTJ areperpendicular the MTJ layers, the effect of the spin torque from thespin current generated by the SHE can be described using an effectivemagnetic field H_(ST). The spin torque per unit moment generated byinjected spin current can be written as

${\tau_{ST} = {\frac{\hslash}{2{eM}_{s}t}J_{S}\overset{\Cap}{\sigma} \times \overset{\Cap}{m}}},$where ℏ, e, M_(S) and t represent Planck's constant, electron charge,saturation magnetization of the FL and the thickness of the FL,respectively, and J_(S) is the spin current injected into the FL fromthe SHE. Meanwhile, the torque generated by a magnetic field in generalcan be written as {right arrow over (σ)}={circumflex over (m)}×{rightarrow over (H)}. By comparing the form of the two torques, the effectivemagnetic field induced by the spin Hall effect has the form

${\overset{\rightharpoonup}{H}}_{ST} = {{- \frac{\hslash}{2{eM}_{S}t}}J_{s}\overset{\Cap}{\sigma} \times {\overset{\Cap}{m}.}}$Therefore, {right arrow over (H)}_(ST) is perpendicular to m ₁ andpoints clockwise or counterclockwise, depending upon the direction ofthe injected spins. If J_(S) is large enough such that |{right arrowover (H)}_(ST)|>0.5H_(an) ^(eff), where H_(an) ⁰ is the maximumanisotropy field that the magnetic film can provide, then {right arrowover (H)}_(ST) will induce a continuous rotation of m ₁. Under theeffect of {right arrow over (H)}_(ST), m ₁ will be switchedcontinuously, without a deterministic final state. To achievedeterministic switching an external in-plane magnetic field {right arrowover (H)}_(ext) has to be introduced, which can be easily provided bythe magnetic dipole field of a magnetic layer placed nearby. Theexternal field can be generated by using one or more magnetic elementsin the device in various configurations. In FIG. 6B, an external fieldin the +y direction is applied as an example. Using m_(z) to representthe z component of m ₁, it can be seen that the state with m_(z)>0 willbecome a stable state because {right arrow over (H)}_(ST) and {rightarrow over (H)}_(ext) can be balanced out with each other while m_(z)<0states are still non-stable because {right arrow over (H)}_(ST) and{right arrow over (H)}_(ext) act in the same direction, causing m ₁ tocontinue to rotate. Therefore, under an applied field in the +ydirection, spins injected in the −x direction can switch m ₁ into them_(z)>0 state. By reversing the writing current direction, spins fromthe SHE will be injected along +x direction, causing m ₁ to be switchedinto the m_(z)<0 state. By using spins injected from the SHE, reversibledeterministic switching is realized.

The current through the SHE layer that is required to effect thedeterministic switching of the FL scales linearly with the effectiveperpendicular magnetic anisotropy field H_(demag) ^(eff) of the FL forthe case where the FL and RL are polarized perpendicular to the plane.If H_(demag) ^(eff) is adjusted to be, for example, ˜1000 Oe or less,which is readily achievable through choice of the FL material, itsthickness and careful thermal annealing strong gating of the SHEswitching current can be readily obtainable with SHE/VCMA thatincorporate MTJ's that have a VCMA of the order of d(H_(demag)^(eff))/dV_(MTJ)≈700 Oe/V, as has been experimentally demonstrated (seeFIG. 7C).

Another application of the present 3-terminal MTJ device design forcombining spin Hall torque with voltage controlled magnetic anisotropyis to employ these effects to achieve new, independent control of thefrequency and amplitude of output power of a spin torque nano-oscillator(STNO). Such a device for generating an oscillation signal based on amagnetic tunneling junction in a three-terminal circuit configurationcan be configured to include a magnetic tunneling junction (MTJ) thatincludes (1) a pinned magnetic layer having a fixed magnetizationdirection in the pinned magnetic layer, (2) a free magnetic layer havinga magnetization direction that is in the free magnetic layer and ischangeable, and (3) a non-magnetic junction layer between the magneticfree layer and the pinned magnetic layer and formed of an insulatormaterial sufficiently thin to allow tunneling of electrons between themagnetic free layer and the pinned magnetic layer. A spin Hall effectmetal layer is provided to be nonmagnetic and includes a metalexhibiting a large spin Hall effect to react to a charge currentdirected into the spin Hall effect metal layer to produce aspin-polarized current that is perpendicular to the charge current. Thespin Hall effect metal layer is parallel to and in contact with the freemagnetic layer to direct the spin-polarized current generated in thespin Hall effect metal layer into the free magnetic layer. This deviceincludes a first electrical terminal in electrical contact with the MTJfrom a side having the pinned magnetic layer; and second and thirdelectrical terminals in electrical contact with two contact locations ofthe spin Hall effect metal layer on two opposite sides of the freemagnetic layer to supply the charge current in the spin Hall effectmetal layer. An oscillator control circuit in this device is coupled tothe first, second and third electrical terminals to supply (1) aconstant current as the charge current via the second and thirdelectrical terminals in the spin Hall effect metal layer to cause aprecession of the magnetization of the free magnetic layer due to thespin-polarized current produced by the spin Hall effect metal layer, and(2) a MTJ junction current directed via the first electrical terminalacross the MTJ to cause a current tunneling across the MTJ thatoscillates due to the precession of the of the magnetization of the freemagnetic layer. This control circuit is configured to adjust the MTJjunction current to control an oscillation frequency or an amplitude ofthe oscillation in the current tunneling across the MTJ.

FIG. 10 shows an example of such an oscillator circuit for exciting anddetecting the magnetic dynamics in the SHE device. Two DC currentsources with common ground can be employed to separately apply currentthrough the SHE strip and through the MTJ. The current through the SHEstrip I_(SHE) injects spin current into the magnetic free layer of theMTJ through the SHE and excites magnetic dynamics therein, while the MTJbias current I_(MTJ) converts the oscillations of the MTJ resistanceR_(rf) arising from the TMR into an oscillating voltageV_(rf)=I_(MTJ)R_(rf) which can then be coupled to a microwave strip-lineor antenna.

In comparison, a conventional 2-terminal MTJ STNO device would have touse the same two terminals to carry the same current as both (1) thedriving current to excite the dynamics and (2) the sensing current toprovide the output power. The 3-terminal SHE/VCMA device in FIG. 10 usestwo separate currents for these functions, respectively, to providebetter technical controls and operation advantages.

FIG. 11 shows the microwave spectra obtained with a prototype SHE/VCMASTNO for different I_(MTJ) while I_(SHE) is held constant. Since thesensing current has little or no influence on the magnetic dynamics,unlike the case for conventional STNO's, the output power P scales asI_(MTJ) ² as shown in FIG. 12A which displays the integrated power P(triangles) of the 3-terminal STNO prototype and its normalized power(circles) vs. I_(MTJ), where T(I_(MTJ)) is the bias-dependent normalizedTMR value of the MTJ. The normalized power is roughly constant withbias.

As illustrated in FIG. 12B, an important aspect of the behavior of thisthree-terminal STNO is a quite significant blue shift of the oscillatorfrequency as I_(MTJ) is increased in the positive direction. This isquantitatively related to the perpendicular magnetic anisotropy changeinduced by the changes in the electric field across the MgO tunnelbarrier as I_(MTJ) is varied. Thus as demonstrated by FIGS. 12A and 12B,the 3-terminal STNO embodiment of the spin Hall effect in combinationwith voltage control magnetic anisotropy enables independent control ofthe magnetic dynamics and the output electric power of a spin torquenano-oscillator, therefore providing a greater and more versatile tuningof the frequency and variation of the amplitude of the output microwavesignal.

In implementing 3-terminal MTJ devices based on two independent controlmechanisms, it is desirable to produce a sufficiently large effectiveperpendicular demagnetization field H_(demag) ^(eff) of the free layeras indicated in Equations (1) and (2) to affect the critical orthreshold spin-polarized current for switching the magnetization of thefree magnetic layer. Various material combinations can be selected forthe MTJ device, including proper transitional metal elements in desiredcrystalline phases. One technique for achieving a sufficiently largeeffective perpendicular demagnetization field H_(demag) ^(eff) of thefree layer is to provide a thin transition metal layer between the freemagnetic layer and the SHE metal layer as shown in the MTJ example inFIG. 13.

In FIG. 13, the material and the thickness of the thin transition metallayer are selected with respect to the material configurations of thefree magnetic layer and the SHE metal layer to enable the interfacingbetween the thin transition metal layer and free magnetic layer toproduce a strong interfacial anisotropy, thus effectuating acontribution to the perpendicular demagnetization field H_(demag) ^(eff)of the free layer and enhancing the voltage-controlled magneticanisotropy (VCMA) effect of the 3-terminal MTJ device. This thintransition metal layer may not exhibit a significant spin Hall effectand is provided between the free magnetic layer and the SHE metal layeras a mechanism to engineer an effective 3-terminal MTJ for dual controlsbased SHE and VCMA effects. The combined structure of the thintransition metal layer and the SHE metal layer can be treated as acomposite SHE metal layer. As a specific example, a 3-terminal MTJdevice based on an in-plane free magnetic layer having FeCoB wasfabricated to include a layer of beta W (4 nm) as the SHE metal layerand a layer of Hf (1 nm) as the thin transition metal layer. Conductedmeasurements of this MTJ device show both strong SHE and VCMA effects.In addition, the thin transition metal layer between the free magneticlayer and the SHE metal layer in FIG. 13 can be used for enhancing theperpendicular anisotropy of the FL layer of MTJ devices where both theFL and PL layers have perpendicular magnetization directions as shown inFIGS. 6A and 6B.

In at least some of the above 3-terminal MTJ examples, the interfacebetween the SHE metal layer and the free magnetic layer of the MTJ iselectrically conductive due to the fact that the either the free layerin direct contact with the SHE metal layer or the thin transition metallayer in FIG. 13 is electrically conducting. Such configurations producea shunt path so that the charge current that is supplied by the secondand third terminals into the SHE metal layer is leaked into the shuntpath. This leaking causes the actual charge current that stays withinthe SHE metal layer to reduce and this reduction, undesirably, reducesthe spin-polarized current generated by the SHE effect. For 3-terminalMTJ devices that use SHE metal layers with high resistivity, thisundesired leaking of the charge current can be significant. To ensureefficient generation of the spin-polarized current in the SHE metallayer, a thin magnetic insulator layer may be inserted between the MTJstack and the SHE metal layer to prevent or reduce leaking of the chargecurrent in the SHE metal layer into the MTJ stack. The thickness of thethin magnetic insulator layer is sufficiently small to allow tunnelingof electrons since a sensing current needs to pass through the MTJ stackfor various circuit operations, including read operations of a MTJmemory cell and generation of oscillation signals in STNO circuitsdescribed above. This thin magnetic insulator layer is a magnetic layerthat reduces the relaxation effect on the spin-polarized current fromthe SHE metal layer generated from the spin Hall effect. This thinmagnetic insulator layer can be a ferromagnetic or ferrimagneticinsulator layer. Various magnetic insulator materials can be used as thethin magnetic insulator layer, e.g., YIG (Yttrium Iron Garnet) andothers.

FIGS. 14A and 14B show two examples of 3-terminal MTJ devicesimplementing the above thin magnetic insulator layer. In FIG. 14A, thethin magnetic insulator layer is placed between the free magnetic layerand the SHE metal layer. In FIG. 14B, the thin magnetic insulator layeris placed between the thin transition metal layer in FIG. 13 and the SHEmetal layer. In both examples, the thin magnetic insulator layer reducesthe leaking of the charge current in the SHE metal layer into the MTJstack and enhances the generation of the spin-polarized current enteringthe MTJ stack.

While this patent document and attachment contain many specifics, theseshould not be construed as limitations on the scope of any invention orof what may be claimed, but rather as descriptions of features that maybe specific to particular embodiments of particular inventions. Certainfeatures that are described in this patent document and attachment inthe context of separate embodiments can also be implemented incombination in a single embodiment. Conversely, various features thatare described in the context of a single embodiment can also beimplemented in multiple embodiments separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described in this patent document and attachment should notbe understood as requiring such separation in all embodiments.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in this patent document and attachment.

What is claimed is:
 1. A magnetic tunneling junction memory device basedon a three-terminal circuit configuration, comprising: an array ofmemory cells for storing data; and a memory control circuit coupled tothe array of memory cells and operable to read or write data in thememory cells, wherein each memory cell includes: a magnetic tunnelingjunction (MTJ) that includes (1) a pinned magnetic layer having a fixedmagnetization direction, (2) a free magnetic layer having amagnetization direction that is changeable, and (3) a non-magneticjunction layer between the magnetic free layer and the pinned magneticlayer and formed of an insulator material sufficiently thin to allowtunneling of electrons between the magnetic free layer and the pinnedmagnetic layer; a spin Hall effect metal layer that is nonmagnetic andincludes a metal exhibiting a large spin Hall effect to react to acharge current directed into the spin Hall effect metal layer to producea spin-polarized current that is perpendicular to the charge current,the spin Hall effect metal layer being parallel to and adjacent to thefree magnetic layer to direct the spin-polarized current generated inthe spin Hall effect metal layer into the free magnetic layer; a firstelectrical terminal in electrical contact with the MTJ from a sidehaving the pinned magnetic layer to receive a gate voltage that modifiesa current threshold of a spin-polarized current flowing across the MTJfor switching the magnetization of the free magnetic layer; and secondand third electrical terminals in electrical contact with two contactlocations of the spin Hall effect metal layer on two opposite sides ofthe free magnetic layer to supply the charge current in the spin Halleffect metal layer; and wherein the memory control circuit is coupled tothe first, second and third electrical terminals to supply (1) thecharge current via the second and third electrical terminals in the spinHall effect metal layer and (2) the gate voltage across the MTJ causinga small current tunneling across the MTJ that is insufficient to switchthe magnetization of the free magnetic layer without collaboration ofthe spin-polarized current flowing across the free magnetic layer causedby the charge current, wherein the memory control circuit is configuredto be operable in a writing mode to simultaneously apply the chargecurrent in the spin Hall effect metal layer and the gate voltage acrossthe MTJ to set or switch the magnetization direction of the freemagnetic layer to a desired direction for representing a stored bit, andwherein the memory control circuit is further configured to be operablein a read mode to apply a read voltage to the first electrical terminalto supply a read current tunneling across the MTJ between the firstelectrical terminal and the spin Hall effect metal layer, withoutswitching the magnetization direction of the free magnetic layer, tosense the magnetization direction of the free magnetic layer thatrepresents the stored bit in the MTJ.
 2. The device as in claim 1,wherein: the memory cells are arranged in rows and columns, the devicecomprises row spin Hall effect metal stripes, each row spin Hall effectmetal stripe being configured to be in contact with a row of memorycells as the spin Hall effect metal layer for each memory cell in therow of memory cells and further coupled to the memory control circuit tocarry a row charge current as the charge current for each memory cell inthe row of memory cells, and the device comprises column conductivestripes, each column conductive stripe being configured to be in contactwith a column of memory cells respectively located in different rows ofmemory cells and further coupled to the memory control circuit to applya row gate voltage as the gate voltage, or a row read voltage as theread voltage, for each memory cell in the column of memory cells.
 3. Thedevice as in claim 2, wherein: the memory control circuit includes: aplurality of first transistors coupled to the column conductive stripes,respectively, one first transistor per column conductive stripe to applythe row gate voltage or the row read voltage to the first electricalterminals of the memory cells; and a plurality of second transistorscoupled to the row spin Hall effect metal stripes, respectively, onesecond transistor per row spin Hall effect metal stripe to connect tothe second electrical terminals to switch on or off the row chargecurrent in the respective row spin Hall effect metal stripe as thecharge current for each memory cell in a corresponding row of memorycells.
 4. The device as in claim 3, wherein: the memory control circuitincludes a plurality of third transistors coupled to the row spin Halleffect metal stripes, respectively, one third transistor per row spinHall effect metal stripe to connect between the third electricalterminals of memory cells in a corresponding row of memory cells and anelectrical ground.
 5. The device as in claim 4, wherein: the memorycontrol circuit is configured to, in reading a selected memory cell, (1)turn on all of the first transistors to apply the row read voltage tothe first electrical terminals of all of the memory cells, (2) turn offall of the second transistors and (3) turn on one third transistor in acorresponding row spin Hall effect metal stripe in contact with theselected memory cell while turning off other third transistors.
 6. Thedevice as in claim 4, wherein: the memory control circuit is configuredto, in writing to a selected memory cell, (1) turn on one firsttransistor coupled to a column conductive stripe in contact with theselected memory cell while turning off other first transistors to applythe row gate voltage to the first electrical terminal of the selectedmemory cell, and (2) turn on one second transistor and one thirdtransistor in one row spin Hall effect metal stripe in contact with theselected memory cell while turning off other second and thirdtransistors.
 7. The device as in claim 1, wherein: the memory controlcircuit includes a first transistor coupled to the first electricalterminal and operable to turn on or off the gate voltage or the readvoltage applied to the first electrical terminal, a second transistorcoupled to the second electrical terminal to turn on or off the chargecurrent in the spin Hall effect metal layer for each memory cell.
 8. Thedevice as in claim 1, wherein: the spin Hall effect metal layer includestantalum or a tantalum alloy.
 9. The device as in claim 1, wherein: thespin Hall effect metal layer includes hafnium or a hafnium alloy. 10.The device as in claim 1, wherein: the spin Hall effect metal layerincludes iridium or an iridium alloy.
 11. The device as in claim 1,wherein: the spin Hall effect metal layer includes rhenium or a rheniumalloy.
 12. The device as in claim 1, wherein: the spin Hall effect metallayer includes osmium or an osmium alloy.
 13. The device as in claim 1,wherein: the spin Hall effect metal layer includes thallium or athallium alloy.
 14. The device as in claim 1, wherein: the spin Halleffect metal layer includes lead or a lead alloy.
 15. The device as inclaim 1, wherein: the spin Hall effect metal layer includes tungstenmetal or a tungsten alloy.
 16. The device as in claim 1, wherein: thespin Hall effect metal layer includes a transition metal or a transitionmetal alloy.
 17. The device as in claim 16, wherein: the spin Halleffect metal layer includes Cu_(1−x)Bi_(x), Ag_(1−x)Bi_(x),Cu_(1−x)Ir_(x), Ag_(1−x)Ir_(x), Cu_(1−x)W_(x), Ag_(1−x)W_(x),Cu_(1−x)Ta_(x), or Ag_(1−x)Ta_(x).
 18. The device as in claim 16,wherein: the spin Hall effect metal layer includes Pd, Mo, Ru, Ir, Au,Pt, or Bi.
 19. The device as in claim 1, wherein: the spin Hall effectmetal layer includes an intermetallic compound with an A15 crystalstructure.
 20. The device as in claim 19, wherein: the metal compoundwith an A15 crystal structure includes Ta₃Al, Nb₃Sn, W₃Ge, Ir₃Hf, TaN,WN or NbN.
 21. The device as in claim 1, wherein: the spin Hall effectmetal layer has a thickness that is less than or equal to five times ofa spin diffusion length of the spin Hall effect metal layer.
 22. Thedevice as in claim 21, wherein: the spin Hall effect metal layer has athickness greater than a spin relaxation length of the spin Hall effectmetal layer.
 23. The device as in claim 1, wherein: the spin Hall effectmetal layer is configured to exhibit a spin Hall angle greater than0.05.
 24. The device as in claim 1, wherein: the pinned or free magneticlayer includes Fe, Co, Ni, or an alloy including Fe, Co or Ni.
 25. Thedevice as in claim 1, wherein: the pinned or free magnetic layerincludes an alloy including Fe, Co or Ni with a non-magnetic material.26. The device as in claim 1, wherein: the pinned or free magnetic layerincludes two or more layers.
 27. The device as in claim 1, wherein: thepinned or free magnetic layer includes two or more layers and each layerincludes (1) Co and Ni, (2) Co and Pt, or (3) Co and Pd.
 28. The deviceas in claim 1, wherein: the insulator material for the non-magneticjunction layer between the magnetic free layer and the pinned magneticlayer is configured to be sufficiently thick to obtain a low currenttunneling through the MTJ and is sufficiently thin to obtain a low gatevoltage for the writing operation or read voltage for the readingoperation.
 29. The device as in claim 28, wherein: the insulatormaterial has a thickness between 1 nm and 2 nm.
 30. The device as inclaim 1, wherein: each of the pinned and free magnetic layers has amagnetization direction parallel to the layer.
 31. The device as inclaim 1, wherein: each of the pinned and free magnetic layers has amagnetization direction perpendicular to the layer, the device furtherincludes a magnetic mechanism to produce a magnetic bias field at thefree magnetic layer and in a field direction parallel to the freemagnetic layer.
 32. The device as in claim 31, wherein: the magneticmechanism includes a magnetic layer that produces the magnetic biasfield and is located between the first electrical terminal and the spinHall effect metal layer.
 33. The device as in claim 32, wherein: eachmemory cell includes a non-magnetic spacer layer in contact with thepinned magnetic layer, and the magnetic layer is in contact with thenon-magnetic spacer layer and configured to have a magnetizationdirection in the magnetic layer to produce the bias magnetic field inthe free magnetic layer.
 34. The device as in claim 1, wherein: the spinHall effect metal layer is in contact with the free magnetic layer. 35.The device as in claim 1, comprising: a transition metal layer incontact with and between the free magnetic layer and the spin Halleffect metal layer to effectuate an interfacial anisotropy at the freemagnetic layer.
 36. The device as in claim 35, wherein: each of the freeand pinned magnetic layers has a magnetization direction parallel to thelayer.
 37. The device as in claim 35, wherein: each of the free andpinned magnetic layers has a magnetization direction perpendicular tothe layer.
 38. The device as in claim 1, comprising: a magneticinsulator layer in contact with and between the free magnetic layer andthe spin Hall effect metal layer to prevent the charge current insidethe spin Hall effect metal layer from leaking into the free magneticlayer and to allow for tunneling of electrons under an applied voltage.39. A method for operating a magnetic memory device based on athree-terminal circuit configuration, comprising: providing a magneticstructure comprising a spin Hall effect metal layer located over asubstrate, a free magnetic layer located over the substrate and adjacentto the spin Hall effect metal layer, an insulating spacer layer locatedover the substrate and contacting the side of the free layer oppositethe spin Hall effect metal layer, and a pinned magnetic layer locatedover the substrate and over a side of the insulating spacer layeropposite the free layer; applying a gate voltage between a firstelectrical terminal in contact with the pinned magnetic layer and thespin Hall effect metal layer to modify a perpendicular magneticanisotropy of the free magnetic layer, without allowing the gate voltagealone to cause switching of a magnetization direction of the freemagnetic layer; applying a lateral current between two electricalterminals in the spin Hall effect metal layer to induce a verticalspin-polarized current into the free magnetic layer without allowing thelateral current alone to cause switching of the magnetization of thefree magnetic layer; and synchronizing the gate voltage and the lateralcurrent to use a combined operation of (1) the vertical spin-polarizedcurrent into the free magnetic layer by the lateral current and (2) themodification of the perpendicular magnetic anisotropy of the freemagnetic layer by the gate voltage to switch the magnetization of thefree magnetic layer to be in a stored magnetization for representing astored bit.
 40. The method as in claim 39, comprising: applying a readvoltage gate voltage between the first electrical terminal in contactwith the pinned magnetic layer and the spin Hall effect metal layer,without allowing the read voltage alone to cause switching of the storedmagnetization direction of the free magnetic layer and without applyingthe lateral current between the two terminals in the spin Hall effectmetal layer, to cause a read current to tunnel through the insulatingspace layer for sensing a resistance between the pinned and the freemagnetic layers in reading the stored bit.
 41. The method as in claim40, comprising: providing memory cells having the magnetic structurearranged in rows and columns; providing row spin Hall effect metalstripes, each row spin Hall effect metal stripe being configured to bein contact with a row of memory cells as the spin Hall effect metallayer for each memory cell in the row of memory cells and operable tosupply a lateral current for each memory cell in the row of memorycells; providing column conductive stripes, each column conductivestripe being configured to be in contact with a column of memory cellsrespectively located in different rows of memory cells and operable toapply the gate voltage or the read voltage for each memory cell in thecolumn of memory cells; in reading a selected memory cell, applying rowread voltage to the first electrical terminals of all of the memorycells without applying the lateral current to any of the secondtransistors while allowing only the selected memory cell to have theread current for reading the selected memory cell; and in writing to aselected memory cell, applying the gate voltage to the first electricalterminals of a column of memory cells including the selected memory cellwithout applying the gate voltage to other columns of memory cells, andsupplying the lateral current in one row spin Hall effect metal stripein contact with the selected memory cell without supplying the lateralcurrent in other row spin Hall effect metal stripes.